The invention is concerned with a picture frame rate conversion system for TV signals and the like. It is particularly suitable for moving picture frame rate conversions, e.g., the mutual conversions between two TV standard systems, and is effective in recovering a time-axis compressed moving picture signal.
A moving picture in general, including the TV picture, is not given by a continuous signal in time. This is because the basic principle of a moving picture is to display a certain fixed number (or frame rate) of still pictures one after another per second.
When it comes to TV systems, for example, Japan's TV system has a frame rate of 30 frames/second, while the TV system adopted in European countries including the U.K. and France has a frame rate of 25 frames/second. Therefore, it is indispensable to have an appropriate frame rate conversion technique for the exchange of TV broadcast programs between countries of different frame rate.
Let us explain the conventional technique used in converting frame rate. A color TV signal in general is composed of three components known as R, G, and B or Y, I, and Q respectively. In converting the frame rate of such a signal, the signal is first decomposed into its three components, and then each component gets a desired frame rate conversion by the same method. Therefore, the conventional frame rate conversion of one of the three components is explained below. This is equivalent to that of a monochrome TV signal.
FIG. 1 and FIG. 2 show the basic principle of frame rate conversion between a frame rate of 25 frames/second and that of 30 frames/second. That is, a conversion frame 25 frames/second ((a).sub.25, (b).sub.25, ---) into 30 frames/second ((a).sub.30, (b).sub.30, ---) is shown in FIG. 1 and the inverse conversion is shown in FIG. 2. In both cases, each of the solid lines and dotted lines drawn in the perpendicular direction to the time axis represent one frame of a moving picture signal that extends two-dimensionally.
As the ratio of frame rates in FIG. 1 and FIG. 2 is 25/30=5/6, if at a time point the position of a frame of 25 frames/sec. and that of a frame of 30 frames/sec. are synchronized, synchronization of the both frame groups takes place at every 5 frame intervals of one or 6 frame intervals of the other. For example, in FIG. 1, starting with the same position of (a).sub.25 and (a).sub.30, the position of (f).sub.25 coincides with that of (g).sub.30, and in FIG. 2, in the same condition the position of (g).sub.30 coincides with that of (f).sub.25. Generally, in a frame rate conversion, a process is repeated with a period determined by the ratio of frame rates (in the above examples, 5 frame intervals of one group or 6 frame intervals of the other group.), and it is sufficient to explain the process for one period.
In FIG. 1, the converted frames (a).sub.30, (b).sub.30, ---, (g).sub.30 are obtained as follows.
frame (a).sub.30 -- frame (a).sub.25 itself, PA1 frame (b).sub.30 -- synthesized from frame (a).sub.25 and frame (b).sub.25. PA1 frame (c).sub.30 -- synthesized from frame (b).sub.25 and frame (c).sub.25. PA1 frame (d).sub.30 -- synthesized from frame (c).sub.25 and frame (d).sub.25. PA1 frame (e).sub.30 -- synthesized from frame (d).sub.25 and frame (e).sub.25. PA1 frame (f).sub.30 -- synthesized from frame (e).sub.25 and frame (f).sub.25. PA1 frame (g).sub.30 -- frame (f).sub.25 itself.
An interpolation process is carried out in synthesizing a converted frame. A device that performs the interpolation process is called an interpolation filter, and in the explanation to follow, the simplest interpolation filter to be realized in hardware, a linear interpolation filter, is considered as an example.
FIG. 3 shows a interpolation process that provides an interpolated frame I from given consecutive frames, i.e., frame A and frame B. That is, each picture element of interpolated frame I (i.e., a sampled value of the sampled part of a picture signal) Y.sub.ij (i: scanning line number, j: column number counted from leftmost) is produced from a picture element X.sub.ij.sup.(1) of frame A and a picture element X.sub.ij.sup.(2) of frame B, each of which has the same position coordinates as those of the picture element y.sub.ij. The interpolation rule is given by the following expression. EQU y.sub.ij =b X.sub.ij.sup.(1) +a X.sub.ij.sup.(2) ( 1)
where a and b are proportional to the distance on the time axis between frame A and frame I and that between frame B and frame I respectively, and are normalized as follows: EQU a+b=1.0 (a, b.gtoreq.0) (2)
For example, in FIG. 1, when frame (c).sub.30 is synthesized from frame (b).sub.25 and frame (c).sub.25, a and b are given as follows. EQU a=2/3, EQU b=1/3
In the example shown in FIG. 3, if the original picture signal is a quiescent picture signal, the signal value of frame A is the same as that of frame B ignoring possible noise superposed, and the signal value of interpolated frame I is equal to that of frame A or that of frame B. Therefore, no deterioration arises in picture resolution. If the original signal is a moving picture signal, however, two picture elements X.sub.ij.sup.(1), X.sub.ij.sup.(2) shown in FIG. 3, hardly have the same signal value, and an extra noise component, known as area error is included in the corresponding interpolated picture element. As a result, the conventional technique has the following drawbacks.
(i) The signal of interpolated frame I gets extremely blurred.
(ii) The movement of a converted moving picture is somewhat unnatural. (this is called jerkiness)
It is clear that these drawbacks more striking with the progress in the TV camera's picture resolution and the increase in velocity the picture's movement. (see "A frame rate conversion of moving picture signals -- an analysis of the conversion characteristic with the time axis direction process." (in Japanese) Trans. of the Institute of Electronics and Communication Engineers of Japan, '84/2, Vol. J 67-B, No. 2.)